In known measuring machines of the portal variety, the means of x-direction guidance of the portal (i.e., against lateral displacement in the y-direction) is operative at one or both feet of the portal. In the case of manually operated machines, the measurement probe is driven by a knob on the z-measurement spindle, and this knob is grapsed by the operator, for control of machine movement in all three axes.
Such measuring machines are of limited precision since, even with careful development of the guide means, machine movement develops flexural moments, particularly on the portal and on the measurement spindle; the magnitude of such moments depends on the instantaneous position of the measurement head and on the acceleration or deceleration forces applied by the operator. Stiffening of the measurement spindle is possible only to a limited degree, since stiffening entails additional mass, and the acceleration and braking of the additional mass per se results in deformation of the portal. Furthermore, machine operation degrades, the greater the masses to be moved.
Bending forces occur also in motor-operated machines if the drive does not act precisely at the center of gravity of the displacement carriages.
U.S. Pat. No. 4,175,327 discloses a measuring machine of portal type in which the portal is moved via a drive which acts on the bridge portion of the portal in the vicinity of its center of gravity. Although transverse forces acting on the portal are reduced by this drive technique, as compared with a one-sided drive, they are not completely eliminated since the center of gravity, as distinguished from the point of application of the drive on the portal, is shifted as a function of y-axis position of the transverse carriage. Since x-direction guidance for the portal is via one of the two feet of the portal, deformation of the portal can occur, even for the case of this patented measuring machine.